Field of the Invention
The present invention relates to a process of diagnosing a state of a mixed-potential gas sensor, and more particularly to a process of diagnosing a deterioration state of a sensing electrode of a gas sensor.
Description of the Background Art
Gas sensors that sense a predetermined gas component in a measurement gas such as an exhaust gas, for example, to determine its concentration come in various types such as a semiconductor gas sensor, a catalytic combustion gas sensor, an oxygen-concentration difference sensing gas sensor, a limiting current gas sensor, and a mixed-potential gas sensor. Some of these gas sensors are obtained by providing electrodes containing a noble metal or a metal oxide as its main constituent to a sensor element mainly made of ceramic is a solid electrolyte such as zirconia.
As is also well-known, in a gas sensor which includes a sensor element mainly made of ceramic such as zirconia, a gas component in a measurement gas or a poisoning substance is adhered to the surface of the electrode due to long-term use, or a composing substance of the electrode is sintered due to exposure of the electrode to a high-temperature atmosphere, so that an output value may vary although a concentration of a gas component to be measured in a measurement gas is constant.
Among the above-stated causes for output change of a gas sensor, adhesion of a poisoning substance and sintering of a constituting material of the electrode are irreversible phenomena, and it is considered to be difficult to cope directly with change in output value due to electrode deterioration (irreversible deterioration) caused by those phenomena.
On the other hand, it is possible to cope with output change due to adhesion (adsorption) of a gas component in a measurement gas to the surface of the electrode, by carrying out a predetermined recovering process and removing the adsorbed gas component. That is, such output change is caused due to electrode deterioration (reversible deterioration) caused by a reversible factor. With regard to a gas sensor subjected to such reversible deterioration, execution of a recovering process would allow an original (initial) output value to be re-attained, or would allow an output value as close to the original output value as possible to be obtained.
Examples of the foregoing recovering process include an electrical process (for example, refer to Japanese Patent Application Laid-Open No. 6-265522 (1994) and Japanese Patent No. 3855979), and a heating process (for example, refer to Japanese Patent Application Laid-Open No. 11-326266 (1999)).
The electrical process is a method for recovering output by alternately applying positive and negative potentials between electrodes that are paired through a solid electrolyte, so as to refine the electrode or to desorb an absorbed substance.
On the other hand, the heating process is a method for recovering output with exposure of an adsorbed substance or a poisoning substance to a high temperature to desorb or burn (oxidize) the substance.
Also, a method of diagnosing presence or absence of an anomaly deterioration for various gas sensors is already known (for example, refer to Japanese Patent No. 4580115, Japanese Patent No. 3855877, Japanese Patent No. 4669369, and Japanese Patent Application Laid-Open No. 2014-48279).
Japanese Patent No. 4580115 discloses a technique for judging presence or absence of an anomaly from a detected value of internal resistance of a solid electrolytic material forming a gas sensor based on an impedance model, in order to prevent a heater from being excessively heated due to an anomaly (increase in resistance) of the solid electrolytic material.
Japanese Patent No. 3855877 discloses a deterioration detection apparatus which includes air-fuel ratio detection means including a solid electrolytic element, means for detecting deterioration of the air-fuel ratio detection means by comparing output values which are respectively provided in cases where different temperatures are set for the solid electrolytic element, temperature control means for controlling a temperature of the solid electrolytic element, and means for detecting a failure in the temperature control means.
Japanese Patent No. 4669369 discloses an apparatus which periodically measures internal impedance of a sensor element, to judge that a failure such as a short circuit or disconnection occurs in the sensor element when a difference value resulted from the periodical measurement exceeds a threshold.
Japanese Patent Application Laid-Open No. 2014-48279 discloses a gas-sensor control apparatus which makes an atmosphere within a measurement chamber included in a detection element of a gas sensor into two different states, measures element resistance in the respective states, and detects presence or absence of deterioration or an extent of deterioration in the detection element based on a magnitude of a difference value between the measured values.
What is a difficult thing in continually using a gas sensor in which the above-stated output change occurs is to judge what timing is proper for carrying out a recovering process. This is because, if a recovering process is carried out more frequently than necessary, sintering of a material forming an electrode becomes conspicuous unfavorably, while reduction in a sensor output which is caused due to adsorption of a gas component in a measurement gas into a surface of the electrode is prevented. This results in an issue of how reversible deterioration of an electrode can be appropriately grasped. The reason is that, as long as a recovering process is carried out promptly in a case where it is judged that an electrode is reversibly deteriorated to an extent that the recovering process is necessary, a deteriorated state is canceled, so that a sensor output which is reduced is recovered.
While each of Japanese Patent Application Laid-Open No. 6-265522 (1994), Japanese Patent No. 3855979, and Japanese Patent Application Laid-Open No. 11-326266 (1999) discloses a recovering process, none of the above-cited patent literatures discloses or suggests how a deterioration state of an electrode is judged with regard to a gas sensor which is continually used.
Also, in a diagnosing method disclosed in each of Japanese Patent No. 4580115, Japanese Patent No. 3855877, and Japanese Patent No. 4669369, and Japanese Patent Application Laid-Open No. 2014-48279, there is neither disclosure nor suggestion about determinatioFIGSn of a timing to carry out a recovering process, based on an extent of reversible deterioration of an electrode.